Hydraulic retarders or decelerators of the hydrodynamic type generally comprise at least one rotatable member (rotor) coupled to a shaft(e.g. the drive shaft of an automotive vehicle such as a heavy truck or other massive vehicle), a stator juxtaposed with the rotor, and a heat exchanger. The rotor generates a circular flow of the hydraulic fluid through compartments in the rotor and stator and, in addition, displaces the fluid through the heat exchanger with the fluid exerting the drag upon the rotor and thereby retaining rotation of the shaft. Because of the conversion of kinetic energy of the vehicle, resulting from the braking or retarding action, the fluid is heated and the heat must be dissipated in the heat exchanger.
The braking effect (or force) is dependent upon the pressure in the hydraulic retarder. Consequently, the hydraulic retarder is usually associated with a control valve having a hand or foot actuated member for regulating the braking effect.
Hydrodynamic decelerators of this type can be used on heavy vehicles operating at high velocities, where pad-type friction brakes are less effective, the vehicles generally also having such friction brakes which can be brought into operation when the speed of the vehicle is reduced, e.g. by a combination of engine braking and the use of hydraulic retarder, since the hydraulic retarder is less effective in terms of braking efficiency and lower speeds.
The braking effect is varied as a function of the applied oil pressure because the friction between the rotor and the hyraulic fluid and between the stator and the hydraulic fluid is directly proportional to the oil pressure. As the pressure is incresed, the braking effect is increased becauase the friction force increases and the generation of heat, resulting from increased retardation, likewise increases.
In water-cooled engines, the dissipation of the heat is effected through a heat exchanger which can utilize the cooling water whereas in air-cooled engines the heat exchanger dissipates the heat to the cooling air.
Thus different degrees of filling of the hydraulic circuit produce different braking effects on the vehicle and it is possible by controlling the filling or, put otherwise, hydraulic pressurization of the decelerator, to vary the braking effect in a stepless manner from a maximum value to zero. The maximum braking moment or torque which can be developed will depend upon the gear which is engaged and hence the transmission ratio between the engine and the wheels. Even if it is in highest gear, the braking effect is significant so that, as a rule, non downshifting is required. In secondary retarders, the retarder torque or moment is equal to the braking moment or torque. The braking force can be limited as required, being a maximum of 1000 Nm on the drive side in conventional systems.
Experience has shown that conventional systems frequently cannot be operated most effectively because the heat exchanger can no longer dissipate all of the heat of the fluid generated by a particular retarding action. The invention is therefore based upon the discovery that both the possible and desirable braking forces can considerably exeed the capacity of the heat exchanger or cooling device used to dissipate the heat evolved and corresponding to the braking effect generated.
When short applications of the hydraulic retarders are utilized, the retarder, the liquid circulating path, and the heat exchanger generally can absorb the heat generated even with the development of very high braking forces. However, with high braking forces and long braking periods, the limited oil volume, the capacity of the heat exchanger and like phenomena detrimentally affect the ability to dissipate the heat which is produced and overheating of the hydraulic fluid can occur. Such overheating can lead to failure of the transmission and/or the engine.